Electronic watches are now capable of providing more than simple time-keeping functions. Many electronic watches can provide such things as detailed information on appointments or other reminders, or even telephone and address information. This information is frequently entered into the watch through buttons on the watch case, a process which is often complex and tedious for the user. A faster and more convenient method of entry is to use a second device, such as a personal computer or Personal Digital Assistant (PDA), to enter the required information, followed by transferring the information to the watch. The difficulty with this method has been in finding a way to transfer the information quickly and reliably from the computer or PDA to the watch.
The small physical size and structure of a watch or similar personal device, such as a pendant timepiece or pager, provides unique challenges in the design of a communication system that can transfer information between the watch and another device. The small physical size of a watch severely limits the space available to house the components needed by the communication system. Moreover, only a limited supply of electrical energy is available at a low voltage and current from the watch battery. A typical watch battery has a nominal voltage of 3V and a current capacity of 150 mAh which provides a life of 2.5 years at a 7 microamp load.
One technique which has been used for the transfer of information to and from a watch utilizes metal contacts on the watch case which mate with connectors coupled to the computer or PDA to permit transfer information into the watch. Unfortunately, such contacts easily become dirty or damaged, causing the transfer to fail, and can expose the connected components to damage by Electro-Static Discharge (ESD).
Information may be transferred to and from a watch using an optical coupling mechanism which consist of an external transmitter having a flashing light and a light sensitive receiver located in the watch. The transmitter modulates the light with the information signal that is being sent to the watch, and the receiver detects the "on" and "off" conditions of the light to capture the data being sent to the watch. Such optical coupling arrangements also have problems: data is sent in one direction only to the watch, so that handshaking operations can't be used to implement desireable error control and syncronization functions, and data can't be sent back from the watch to an external data utilization device. Further, optical couplers are susceptible to background light interfering with the transmitted light; and typically operate at low data rates of only a few tens of bytes per second.
Existing and conventional bidirectional infrared communication systems have significant advantages over physical connections and unidirectional optical couplers Unfortunately, infrared communication mechanisms have large power consumption, require expensive additional components, or both. For example the IrDA Serial Infrared Data Link Standard promulgated by the Infrared Data Association for use with notebook computers, mobile phones, and other handheld devices communicating over distances up to 1 meter, requires an encoder and decoder interface to a UART and an IR transducer. The power requirements for such a system, while acceptable for the larger battery capacity available in a handheld device, would quickly drain a watch battery.
What is needed, therefore, is a low-power two-way communication link between a watch or other small electronic device and another electronic device such as a personal computer. This link would ideally use as few components as possible due to the small physical space available in a normal watch housing, add little to the cost of the system, and provide two-way communication so that stored information can be read from the watch, new information or commands to be loaded into the watch, and transmit acknowledgment messages to indicate whether prior messages were received correctly.